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Fig. 6.12 Rather wide barchans
in the United Arab Emirates,
viewed from a kite camera. This
barchan field is at the edge of an
area where the sand is more
abundant, and thus grades into
the distance into more continuous
barchanoid ridges. Photo R.
Lorenz
actually blows along the length of the dune (i.e., because of
the vector addition, the instantaneous sand transport, in the
instantaneous wind direction, is never the same as the
resultant sand transport).
Because the winds are generally somewhat orthogonal to
the crest, there may be some net sideways migration of at
least the crest, if not the whole dune. The large size of
linears means this migration, however, is generally very
small (although may be indicated in the structure of the beds
within the dune).
Much of the older literature discusses helical vortex
winds as a means of forming linear dunes. It is certainly true
that roll vortices can form (with their diameter determined
more or less by the thickness of the atmospheric boundary
layer), and the presence of these vortices can often be
revealed by lines of cloud that form at the upwelling sides.
The presence of linear dunes may help to stimulate and/or
anchor these dunes. However, the extension to these vorti-
ces as having any importance at all in sand transport, and
thus in defining and forming linear dunes, has never been
demonstrated. That said, the boundary layer thickness may
be what defines the ultimate height, and therefore spacing,
of linear dunes (e.g., Andreotti et al. 2009; Lorenz et al.
2010) as well as the size of roll vortices, so the vortices and
linear dunes may have a common controlling parameter.
But that is not the same as saying one controls the other.
The Namib and the Arabian deserts have perhaps the best
examples of giant linear dunes, the latter with perhaps the
most perfectly uniform examples (see Figs. 6.18 and 6.19 ).
Much smaller—and usually partly vegetated—linear dunes
can be found in the USA, the Kalahari desert (Fig. 6.20 ) and
the Simpson desert in Australia. Titan's massive sand seas
are almost exclusively of linear dunes (Fig. 6.21 ), yet linear
dunes are almost entirely absent on Mars.
When the bimodal wind regime that forms linear dunes
changes, the dune system re-orients itself (see Chap. 7 ) .
This ultimately may lead to a disappearance of the dunes
(perhaps seen in Fig. 6.22 ), more typically recognized via a
conversion into a megabarchan form (e.g., Fig. 6.23 ) per-
haps seen on Titan (Fig. 6.24 ).
6.6
Reversing
Symmetric to asymmetric sand ridges (depending on the
relative intensities of the winds) having two slip faces, one
each on both sides of the ridge crest. In a sense, these can be
seen as an end member of linear dunes. Reversing dunes
form primarily through vertical accretion of sand in
response to a bimodal wind regime, although one wind
direction can at times be stronger than the other, resulting in
an asymmetric profile. Correspondingly, barchans that are
 
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